Turbines are used in many fields for driving generators or process machines. In this case, the energy content of a flow medium, for example a combustion gas produced by combustion of a fuel, is used to produce a rotary movement of a turbine shaft. In this case, a number of rotor blades, which are normally combined into blade groups or blade rows, are arranged on the turbine shaft in order to produce the rotary movement of the turbine shaft, and drive the turbine shaft by way of an impulse transfer from the flow medium. Furthermore, stator blade rows which are connected to the turbine housing are normally arranged between adjacent rotor blade rows in order to guide the flow medium in the turbine unit.
In addition to a large number of other operating parameters, reliable monitoring of what is referred to as the radial gap is important for operation of such a turbine. The expression radial gap in this case refers to the distance between the free end of each rotor blade and the inner housing, which surrounds it, of the turbine. On the one hand, there is a design aim in order to achieve particularly high turbine efficiency for the radial gap to be kept as small as possible so that the flow medium which is guided in the turbine passes through the rotor blades rows with energy being converted and does not flow past the rotor blades through the radial gap, without any energy being transferred. On the other hand, however, it is absolutely essential to preclude any direct contact between the rotor blade end and the inner housing of the gas turbine, for operational safety reasons, in all cases. In this case, it should be remembered in particular that a thermal increase in the length of the rotor blades can occur during use in gas turbines owing to the comparatively high operating temperatures of up to 1200° C. which may possibly occur there, since this leads to the radial gap becoming smaller than when at rest. The radial gap in turbines is thus normally measured and checked regularly, or at least on a sampling basis.
It is therefore desirable to provide concepts for measurement and monitoring of the radial gap in a turbine. These concepts should be designed such that no contact is made in order to avoid adversely affecting the friction-free movement of the turbine. What is referred to as a triangulation measurement may be used as a noncontacting concept for this purpose, in which the radial gap, or in general the distance between a component which is moved past a reference surface and that reference surface, is detected by an optical device. In the case of a triangulation measurement such as this, two light beams are transmitted from the reference surface at an angle to one another such that, in the projection of the plane which is defined by the movement direction of the component and the normal to the reference surface, they intersect at a distance that is more than the maximum expected distance to be measured behind the reference surface. The beam path of the light beams thus forms a triangle in the projection, with a section of the reference surface as the base, in which case the beam path may in particular be chosen such that this triangle is an equilateral triangle. The measurement is in this case carried out by monitoring each of the light beams to determine whether the component to be measured, for example its leading edge, is passing through the respective light beam. This may be measured, for example, in the form of a light barrier or via a reflection signal associated with the respective light beam.
This concept thus makes it possible, with two light beams, to determine the time at which the component interrupts or enters the beam path. If the geometry of the beam path is known and the movement speed of the component is known, the measured time difference between the component passing through the light beams allows a characteristic length to be determined which the component has travelled between the light beams, and this characteristic length can in this case be converted, on the basis of the beam paths, to a distance between the component and the reference surface.
However, reliable use of a concept such as this for measurement of radial gaps in turbines is normally dependent on a comparatively high level of calibration complexity and, furthermore, the achievable accuracy is only limited in this case. Furthermore, vibration that occurs during operation of the turbine may considerably adversely affect the measurement accuracy, so that the reliability of such systems is only limited.